1. Field of the Invention
The present invention relates to an image processing apparatus, and more particularly to an image processing apparatus that compensates deterioration in an image quality upon printing.
2. Description of the Prior Arts
In recent years, printers have spread to offices in general and homes. The improvement of their performance has been remarkable. Printers are classified into several types according to its printing method, among which an electrophotographic printer has a feature of high image quality and high productivity.
In this electrophotographic printer, toner is adhered onto an electrophotographic latent image obtained on a photosensitive member according to ON or OFF of a light emitting device such as a laser diode for developing the latent image, and the developed image is transferred to be fixed on an output medium such as a paper sheet or an OHP sheet, whereby a desired printed-out image can be obtained.
However, deterioration in image quality upon transferring is pointed out as described in Japanese Unexamined Patent Applications Nos. HEI 10-65917, HEI 10-65918, HEI 10-65919 and HEI 10-65920. Such deterioration in image quality is roughly classified into three groups, each of which has a different deterioration phenomenon.
A first phenomenon is, as shown in FIG. 10A, that a density of a half-tone density area is decreased at the boundaries of areas of both half-tone density area and background area (normally non-printed) so that an outputted image varies from the half-tone density area to the background area in a slow-scanning direction.
A second phenomenon is, as shown in FIG. 10B, that a density of a half-tone density area is decreased at the boundaries of areas of both half-tone density area and high density area so that an outputted image varies from the half-tone density area to the high density area in a slow-scanning direction.
A third phenomenon is, as shown in FIG. 10C, that a density of a half-tone density area is decreased, in a fast-scanning direction, at the boundary of areas of half-tone density area of an outputted image.
In FIG. 10, a rectangle in which a dot density is high represents a high density area, a rectangle in which a dot density is low represents a half-tone density area and a white rectangle represents a background area, while an area framed by a bold line represents an area where the aforesaid phenomena are generated. Further, FIGS. 11A and 11B are sectional views of FIGS. 10A and 10B respectively and FIG. 11C is a sectional view of FIG. 10C in the fast-scanning direction. A hatched portion represents-a density decreasing amount in FIG. 11.
From the viewpoint of preventing the image deterioration upon printing, Japanese unexamined Patent Applications Nos. HEI 10-65917, HEI 10-65918 and HEI 10-65919 disclose techniques for reducing the aforesaid image deterioration of three types by compensating an image beforehand, while Japanese Unexamined Patent Application No. HEI 10-65920 discloses a technique that a sample image is printed out to set a compensating parameter upon compensating image data, to thereby be capable of easily determining the compensating parameter.
However, in the image processing apparatus disclosed in each Application, compensation processing that requires a great number of reference areas is performed to a raster image, to thereby entail an increase in cost due to an increase of the amount of hardware in the case of executing the processing with hardware. Further, it causes a problem of bringing an increase in processing time and an increase of a used memory area in the case of executing the processing with software. Additionally, it is not necessary to execute the compensation processing in all pages. In the case where a page is formed by a character having the maximum density and a non-printed background, for example, the compensation processing is not required to be executed. However, the image processing apparatus disclosed in each Application does not find the area which should be subject to the compensation processing beforehand, so that it is necessary to judge whether the compensation processing should be performed over the entire area in a page or not as well as to execute compensation processing to the area that is judged to be subject to compensation processing, thereby entailing an increase in processing time.